1. Field of the Invention
The present invention relates to a vehicle seat, and in particular, to a vehicle seat which, by the cushion material of a seat back, absorbs impact force at the time when an automobile receives a large, forwardly-directed impact force due to a collision from the rear (hereinafter called “rear collision”) or the like.
2. Description of the Related Art
When an automobile receives a large, forwardly-directed impact force due to a rear collision or the like, the impact force is absorbed by the seat backrest which is formed of urethane and has a damping characteristic. At this time, the seated occupant sinks into the seat backrest such that that seat backrest elastically deforms. Thus, the majority of the impact force temporarily accumulates in the seat back as elastic energy. The seated occupant is pushed forward by the temporarily accumulated elastic energy.
To address impact forces caused by rear collisions, Japanese Patent Application Laid-Open (JP-A) No. 11-70029 discloses a seat which absorbs impact force. The seat back of this seat has a pair of side frames which are formed at the respective outer side portions in the vehicle transverse direction and which extend vertically, and a plurality of S-shaped springs which span along the vehicle transverse direction at the vertical direction intermediate portion of the pair of side frames. Further, a lath metal mesh spans between the pair of side frames at the vertical direction intermediate portion of the pair of side frames. This lath metal mesh is provided at the rear of the S-shaped springs so as to be separated by a predetermined distance rearwardly from the S-shaped springs, and such that the longitudinal directions of the openings of the lath metal mesh are directed in the vertical direction.
When a great impact force is applied to this seat, a difference in accelerations arises between the seat and the vehicle occupant due to the inertia of the vehicle occupant, and the seat moves relatively in the advancing direction. Thus, the plurality of S-shaped springs elastically deform greatly toward the rear, and the seated occupant collides with the lath metal mesh via the S-shaped springs and a pad and the like. When the lath metal mesh receives a load in the direction of thickness thereof, it is easy for the lath metal mesh to plastically deform in the surface direction thereof. Thus, due to the reaction force which is directed rearward from the vehicle occupant and is based on the aforementioned impact force, i.e., due to the load applied to the lath metal mesh in the direction of thickness thereof, plastic deformation occurs smoothly at the lath metal mesh. In this way, the energy based on the impact force is absorbed sufficiently, and the shock applied to the seated occupant can be mitigated sufficiently.
However, in the above-described conventional seat, up until the time when the seated occupant collides with the lath metal mesh via the S-shaped springs and the pad and the like, elastic energy caused by the rear collision accumulates in the S-shaped springs and in the springs of the cushion pad of the backrest. This elastic energy becomes energy which pushes the seated occupant forward. Further, although the lath metal mesh plastically deforms in the vertical direction when the seated occupant collides with the lath metal mesh, there is little plastic deformation at the rear of the seat. Thus, the kinetic energy of the seated occupant is not absorbed by the lath metal mesh. Accordingly, a problem arises in that the load of the impact force on the seated occupant is great.